1. Field of the Invention:
This invention relates to a steel sheet of high workability for making cans, cans having a high degree of corrosion resistance and a method of making them. More particularly, it relates to a laminated steel sheet composed of a thin sheet of steel, a layer of a thermoplastic resin and a foil containing aluminum, cans formed essentially from any such laminated steel sheet and a method of making them.
2. Description of the Prior Art:
Cans for holding beer or other carbonated drinks are usually made of an aluminum alloy or tinplate by a combined process of drawing and ironing. They are the so-called drawn and ironed cans. Tinplate is a sheet of steel having an electrocoated layer of tin. This layer usually has only a minimum thickness obtained by employing 2.8 to 5.6 g of tin per square meter, since tin is expensive. A drawn and ironed can requires a strong force for its manufacture, as opposed to a soldered can. The drawn and ironed can has a wall thickness which is as thin as about one-third of the original thickness of tinplate. The severe work of drawing and ironing forms numerous defects in the coated tin layer. An organic protective coating is, therefore, essentially required on the inner surface of a drawn and ironed tinplate can to ensure its corrosion resistance to the drink which it holds. It is usually necessary to repeat the application and baking of any such protective coating twice in order to eliminate the coating defects completely. They are, however, usually done only once for an aluminum can, since aluminum has a relatively high degree of corrosion resistance and an aluminum can is, therefore, satisfactory in corrosion resistance even if there may remain some coating defects.
There are also known drawn and ironed cans which are not made of tinplate, but are formed from a steel sheet coated with a specific organic film. They are disclosed in, for example, Japanese Laid-Open Patent Specifications Nos. 94585/1979 and 174242/1982.
There is also known a laminated material which is composed of two metal sheets and a resin layer disposed therebetween. For example, Japanese Laid-Open Patent Specification No. 036575/1978 discloses a method which produces a laminated metal sheet by forming a resin layer on a steel sheet and laying an aluminum sheet on the resin layer. This laminated sheet is, however, not intended for use in making cans. The specification states that it is intended for use in the field of electrical appliances and provides an inexpensive decorative material. Insofar as it is intended for decorative use, the aluminum side of the laminated sheet having a better appearance is used to form the outer surface of any product thereof.
The protective coating repeated twice on the inner surface of a tinplate on account of thin plating of tin as described above can necessarily results in an increased cost of can production. The cans not made of tinplate, but formed from a steel sheet coated with a organic film are not satisfactory in corrosion resistance.